The text in this background section of the specification is intended to provide understanding of the background of the invention without regard to what may be prior art with respect to the invention. There is no intention to specify what is prior art in presenting this background material.
Zeolites are natural or synthetic hydrated aluminosilicates with an open three-dimensional crystal structure in which water molecules are held in cavities in the crystal lattice. The water can be driven off by heating the crystalline material and the dried zeolite crystal structure can then absorb other molecules of suitable size. Zeolites, with their unique porosity and high surface area, have been widely used in many industrial technologies, including gas adsorption, ion exchange, separation of constituents in fluids, and in catalysis. For this reason they are often called molecular sieves. But, thus far, aluminosilicate molecular sieves have not been usable in the treatment of exhaust gas constituents (CO, HC, and NOx) flowing from a vehicle internal combustion engine. Aluminophosphate molecular sieves also exist in wide crystal structural diversity, but are of limited value in catalysis due to their low acidity and the lack of chemical reduction-oxidation (redox) properties.
More recently, silicoaluminophosphate (SAPO) analogs of aluminophosphate molecular sieves, synthesized by introducing Si (or silica) in place of a portion of the phosphate, have been adapted to provide catalytic properties for acidic and redox catalysis. These silicoaluminophosphate compositions can be formed into different complex crystal structures (SAPO-n), defined by varying proportions of tetrahedral oxides of SiO2, AlO2−, and PO2+, where n denotes a particular framework type. The different crystal structures are each characterized by several interconnected cage-like structures with openings that are smaller than the boundaries of the apparent cage surfaces. Negative charges occur within SAPO frameworks when there are more aluminum atoms than phosphorus atoms within the framework. These negative charges are typically balanced by H+ cations (i.e., positively charged ions) as the SAPO materials are synthesized and calcined; this form is usually referred to as H/SAPO-n.
H-SAPO-n materials are often prepared through hydrothermal crystallization processing of suitable proportions of water-dispersible precursors of SiO2, Al2O3, and P2O5, which yields the negatively-charged crystal structure, defined by arranged Si, Al, P, and O atoms with sufficient H+ cations for a neutral powder product. A structure directing agent, such as an amine or ammonium compound, may be added to the precursors and ultimately decomposed during the thermal processing. It is found that, based on the proportions of the respective four atomic constituents and the structure directing agent; several different silicoaluminophosphate crystal structures may be formed. As stated, they are designated with numbers (SAPO-n) and present different sized cages and openings. In some applications it is desired to replace the hydrogen cations with cations of a metal (M) in one or more subsequent processing steps. And such metal-cation containing SAPOs are often designated as M/SAPO-n.
Of the several different M/SAPO-n crystal structures, Cu/SAPO-34 is of interest as a potential catalyst material in the selective catalytic reduction of NOx with ammonia in the exhaust of diesel engines, or lean-burn gasoline engines, on motor vehicles. SAPO-34 has a crystal structure like that of its naturally occurring aluminosilicate analog, Chabazite, Ca2[(AlO2)4(SiO2)8].13H2O. But the multi-step process of forming SAPO-34 and subsequently replacing its H+ cations with cations of a transition metal, like copper, is expensive. Further, there has been no easy way of controlling the amount of copper cation loading through the small pore openings in the H/SAPO-34 structure. There is a need for a process of forming Cu/SAPO-34 or other M/SAPO-34 materials without having to insert the metal cations into a previously formed H/SAPO-34 caged crystalline structure.